Dual energy x-ray imaging systems may use images that are created at different x-ray energies, in order to distinguish between materials of different atomic composition. Applications of dual-energy x-ray imaging systems may include, but are not limited to, bone densitometry, explosive detection, and quantitative CT (computed tomography).
In these systems, x-ray measurements at two energies may be used for selective material imaging. This approach is made possible because x-rays undergo different types of interactions with matter, at different energies. In the diagnostic range of x-ray energies up to 200 keV, x-rays interact with matter primarily through the Compton and photoelectric interactions. These two types of interactions depend differently on the energy of the incident x-rays: the cross-section for Compton scattering is proportional to the electron density of x-ray target material, while the photoelectric cross-section is proportional to the electron density times the atomic number (Z) cubed. By separately measuring x-ray attenuation at low and high energies, therefore, the Compton and photoelectric interactions can be independently measured. The results of the measurements depend on the type of the x-ray target material, not on the thickness or density of the target material.
A number of methods have been used to carry out dual energy x-ray imaging. One method uses monoenergetic sources, such as radionuclides. The use of radionuclides can lead to very long scan times, due to the low output of the sources. In bone densitometry, this may lead to motion artifacts. A second method approximates monoenergetic x-rays using kVp switched x-ray tubes. Since the measurements at low and high kVp settings are taken at different times, this method can also produce motion artifacts. Also, switching x-ray tube voltage at high frequencies can be technically complex.
A third method uses a single x-ray exposure and energy selective detector. Various configurations of a so-called “front-back detector” have been proposed in the past. Typically, a front-back detector is a two-layered solid state detector, with two scintillator elements, each coupled to a separate sensor, and positioned one in front of another. Both elements are in the path of the x-rays, so that the first element may be more sensitive to low energy x-rays (or all x-rays), and the second elements may sense hardened higher energy radiation passing through the first layer. An additional x-ray filter material, typically 0.6 mm copper, may be introduced between the elements, so that the energy separation between the low energy x-rays and the high energy x-rays is increased. In this configuration, losses in x-ray flux may occur, due to x-ray absorption in the sensor material, the substrate, and the x-ray filter material, increasing noise in the image. Also, direct conversion of x-rays to electrical signal in the photo-sensing material may occur, also increasing noise in the image.
For these reasons, an improved dual energy x-ray detector is desirable.